There are a number of approaches to trying to 'get the whole system in the room'. These are often collectively known as 'Large Group Interventions' or Collaborative Inquiries. Some links and information about some of the more popular can be found below.
Appreciative Inquiry
Appreciative Inquiry is a form of collaborative inquiry which encourages people to discover the positive core of their organisation and then to co-construct the future of the organisation based upon what is best in it.
Future Search
Future Search was developed by Marvin Weisbord & Sandra Janoff. It uses a three-day conference with about 60-70 participants to go through a structured process of imagining the future and finding common ground.
Liquid Café
Liquid Café can be thought of as a hybrid of World Café and Open Space. I 'invented' it for a client but I think that it might have a wider appeal.
Open Space
Open Space was invented by Harrison Owen. It is a process which enables large scale meetings (anything from 20 to 500 participants). The process is very participative, with the agenda being created on the day by those who are present.
Whole Scale Change
Whole-Scale Change is a participative process, which can involve large numbers of people.
World Café
World Cafe is an innovative process which permits people to have conversations around a theme while sitting together at small tables.
Liquid Cafe
Liquid Cafe is a kind of hybrid of Open Space & World Cafe. It is useful when there are a number of topics which need to be explored.
Participative Design
Participative Design was developed by Fred and Merrelyn Emery. Steve Cabana offers a good introduction, as does the IAF site.